Merge PR #1988: #1507: web start-up treats only NotFoundError as a missing file

apps/lightcraft-web/src/files.rs: keeps #1987's and #1988's tests
docs/whats-new.md: keeps both 2026-10-15 entries

docs/web.md

@@ -123,6 +123,11 @@ are the same in every version, so serve them with `Cache-Control: no-cache` (not
     writes, so commands report `saved in memory but not written`, the top bar shows the unsaved
     warning (as on the desktop, see `docs/control-protocol.md`), and saving is retried every
     2 s until it works (`web.stats` → `saveError`).
+  - A failed read at start-up is not taken for a missing file: only `NotFoundError` means absent.
+    When a stored library file (or the `active-library` pointer) exists but can't be read, the
+    library isn't opened and nothing is saved, so the stored copy stays as it was; the window says
+    which file failed (issue #1507). A thumbnail index that can't be read keeps the stored
+    thumbnails instead of deleting them as unindexed.
   - A picked or dropped photo whose bytes can't be stored is not added (it would be gone after
     a reload); a notice says why.
   - One tab at a time: the page holds a Web Lock (`navigator.locks`, `lightcraft-library`); a

docs/whats-new.md

@@ -8,6 +8,12 @@
   now refuses to start and says how many changes are unsaved and why. LightCraft retries the save with every change;
   back up once nothing is left unsaved.
 
+### Web: an unreadable library is not opened
+- In the browser version, a library file that exists but can't be read at start-up is no longer taken for a missing
+  one (issue #1507). LightCraft used to open the library without it, showing an incomplete catalog that the next save
+  would write over the stored copy. It now leaves the library unopened, saves nothing and says which file couldn't be
+  read; reload the page to try again.
+
 ## 2026-10-14
 
 ### Model downloads time out on a name that doesn't resolve